1. Field of the Invention
This invention relates generally to medical procedures and appliances used in diagnosis or therapy; and more particularly to procedures and apparatus for determining flow rate of blood in a living body.
2. Prior Art
At present, cardiac output is most often measured by the thermodilution method. This process requires starting a right-heart catheter in a vein (jugular or subclavian).
The catheter tip is advanced to the vena cava through the right atrium and ventricle, and finally placed in the pulmonary artery. Generally this process is facilitated by floating the catheter tip into position using a balloon, approximately one-and-a-third centimeter in diameter, attached to the catheter tip.
The catheter has a lumen running to the right atrium region, with a port in that region. This port allows rapid infusion of a bolus of cold liquid, ordinarily three to ten cubic centimeters of room-temperature or iced saline solution (or five percent dextrose in water), which mixes with the blood flowing through the right ventricle. Mixed blood and bolus then flow out the pulmonary artery.
The temperature of the resulting mixture of blood and added liquid leaving the ventricle is depressed--relative to the initial temperature of the blood alone--by the cold bolus. Temperature in the pulmonary artery is then measured with a thermistor carried on the catheter about three and a half centimeters from the catheter tip.
Cardiac output is calculated from the temperature drop in the pulmonary artery following cold-bolus injection. The duration of the temperature transient ranges from six to about thirty seconds, depending on the patient's cardiac output. The area under the temperature-transient curve is inversely proportional to cardiac output; therefore to calculate the flow it is necessary to divide an empirical constant by the area under the curve.
Injected dye is sometimes used in place of an injected cold bolus. When dye is used, arterial blood is withdrawn slowly. The concentration of dye in the withdrawn blood is then measured as a function of time and used to compute cardiac output.
It has also been proposed to perform a modified type of thermodilution by adding heat to the blood flow and measuring the resultant temperature rise. Heat can be added through operation of a resistive heater on a catheter, without liquid infusion, as suggested by Philip et al. in "Continuous Thermal Measurement of Cardiac Output," IEEE Transactions on Biomedical Engineering, volume BME-31 No. 5, May 1984, at 393-400.
Other techniques sometimes used are known as the Fick method and the ultrasound method. In the Fick technique, three measurements are necessary: measurement of arterial and mixed-venous blood-oxygen content, and measurement of the rate of oxygen consumption.
The Doppler ultrasound technique measures cardiac output noninvasively. A Doppler probe operating at about five megahertz is placed in the suprasternal notch (the indentation beneath the "Adam's apple") and directed at the ascending aorta. The velocity of the blood thus measured is multiplied by the estimated cross-sectional area of the aorta lumen. The result is an estimation of flow; the estimate can be averaged to determine cardiac output.
All these methods are subject to important drawbacks. Thermodilution and its dye variations typically require the clinician to inject three to ten cubic centimeters of liquid. This is inconvenient, intermittent, and can result in excessive fluid infusion into the patient.
Excessive infusion in turn can increase risk of congestive heart failure or kidney failure in certain patients. Because of this problem, frequent automated injection of fluid is precluded--even though the physician would like to know cardiac output on a minute-to-minute basis. In addition the injection of fluid increases risk of infection.
In addition, the dye variant poses yet another problem. Some patients seem to have exhibited hypersensitive reactions to the material used as dye.
The Philip heating technique poses a problem of potential injury to the patient. As explained in the paper by Philip et al., investigators have yet to establish the maximum permissible temperature rise in any significant fraction of a patient's blood volume; but preliminarily that permissible rise appears to be rather narrowly constrained. Philip and his coauthors predict "feasibility in most mechanically ventilated patients, but difficulty in spontaneously ventilating, more active patients"--and then go on to conclude that "technical difficulties of developing a clinically practical system are formidable . . . ."
The Fick method requires the patient to be in a steady state in which cardiac output does not change significantly on a minute-to-minute basis. This constraint is not a realistic one for surgical cases of practical importance. In addition, the Fick method not only requires precise measurement of arterial and venous blood oxygen, using blood samples, but also requires very expensive or inconvenient instrumentation for measurement of oxygen consumption.
The method is thus cumbersome and expensive. Hillis et al. present a fuller discussion in "Analysis of Factors Affecting the Variability of Fick Versus Indicator Dilution Measurements of Cardiac Output," The American Journal of Cardiology, volume 56, at 764-68 (Nov. 1, 1985).
The Doppler method is sensitive to orientation of the probe. It also requires an estimate of the cross-section of the aorta lumen, using either M-mode ultrasound or a statistically based empirical table. In either case the area estimations are troublesome and inaccurate. More details appear in, e.g., "Non-Invasive Ultrasonic Cardiac Output Measurement in Intensive Care Unit," Levy et al., Ultrasound in Medicine & Biology, volume 11 number 6, at 841-49 (1985).
In summary, the prior art leaves much to be desired in the convenient and continuous measurement of cardiac output.